Wood pulp preparation



Feb. 11, 1958 A. N. PARRETT 2,823,121

woon PULP PREPARATION Filed May 27. 1952 REF\NING OF SUL.F\TE PULP AT [60 c.

[J 97 (I) 9 3 96 l h] 0 95 E Q i a 79 770 Z Z 15 Bi 62 I! m 8 2 so I 9 58 II CD 5Q 6 3 D J 8 o 0 2o 40 so so I00 PERCENT OF TOTAL SODIUM AS SODIUM SULFIDE me so so 40 2o PERCENT OF TOTAL SODIUM AS SODlUM-CARBONATE INVENTOR United States Patent Incorporated, Shelton, Wash, a corporation of Delaware Application May 27, 1952, Serial No. 290,209 7 Claims. (CI. 92-13 This invention relates to Wood pulp preparation and has for its object the provision of an improved method of refining sulfite pulp for the production of high-alpha cellulose in high yield. The invention produces highalph'a cellulose characterized by good brightness, improved reactivity in acetylation processes such as the acetate process and by giving improved rayon yarn or cord in the viscose process.

In accordance with the invention, a chlorinated and washed pulp from the acid sulfite digestion of wood is subjected to digestion at a temperature of at least 140 C. witha digestion liquorconsisting of sodium carbonate (Na CO or sodium sulfide (Nags), or mixtures of both in all proportions, When the sodium present as sodium sulfide is present in amounts in excess of 70% of the total eod-ium, if the liquor is subjected to evaporation and combustion for recovery of heat and chemicals, the resulting smelt will not have a sufiiciently low fusion point. In such case the sulfide content would preferably be dimin'ished by resorting tea carbonation and strippingoperation prior to the evaporation and burning. When it is desired to produce a digestion liquor advantageously amenable to direct recovery of chemicals without diminishing the sodium sulfide content, I use a mixture of sodium carbonate and sodium sulfide in which the gs is present in amounts to provide up to 70% of the sodium of the iniitt'ure.

In its more complete aspects, the invention provides an improved process for the alkaline refining of chlorihated and washed acid sulfite wood pulp with sodium carbonate and sodium sulfide at relatively high temperahires, and the economical recovery of chemicals from the digestion liquor for reuse in alkaline refining. The alkaline refiningis so carried out as to give a waste liquor of such composition that the content of inorganic chem; icals may be readily recovered in useful form from a smelt obtained by evaporation and burning of the waste liquor. v I

The smelt produced from the improved alkaline refining process is characterized by having a low fusion point so that it can be readily removed from the furnace while operating the furnace at a temperature low enough to avoid excessive volatilization of soda salts therefrom. The low melting smelt obtained may be dissolved in water and after suitable clarification used directly in making up digestion liquor for the alkaline refining. By direct use of the dissolved smelt solution, cumbersor'ne processes for converting smelt to other chemicals are eliminated. By providing a cyclic process for alkaline refining of sulfite pulp and recovery of refining chemicals for re-use there results a high degree of economy as well as a minimum of pollution problems.

In simple operations consisting of evaporating and furnacing the efliuent, the soda chemicals are readily recovered from the furnace as a low melting point smelt.

N 2,823,121 Fatented reb. 11, 1958 A. s(')lutionv of thissm'elt, after clarifying and providing suitable makeup or adjustmentfor chemical losses, may be directly re-us ed for the alkaline refining. g

Sodium carbonate-sulfide mixtures in whichthe so,- dium sulfide content represents more than 70% of the sodiuni may however also be practically usedin my processfor the alkaline refining of sulfite pulp. In such case, I employ 'priorto evaporation, and turnacing, a simple pre treatmentof the effluent liquor, later described in detail, whereby the sulfide content of the efliuent is reduced. Ihesulfide so eliminated isrecovered in usable form as concentrated hydrogen. sulfideand the treated liquor with diminished sulfide content will have the same or greater advantage in subsequent evaporation and furnacing as would irectly thiel emuem froin refinin"g' with sodium c'arbonate-"s'ulfi de mixtures of low sulfide content. I In the sulfite pulping process, a lignocellulose material is generally cooked in a solution ofsulfurous acid, part ofthe sulfurous acid being co'rnbined as bisulfite. The cation. cpmbined thus with the bisulfite ion is generally knownas the fpulpii ng base? and this term isused in this sense herein. lful'ping base cations normally used include calcium, sodium, arn'rnoniu'm, and magnesium. Pulp produced by acid sulfite digestion of wood, irrespective. of the. pulpingfbase is commonly termed s'ulfite; 'lpfan'd the ter m is used with this meaninghere in.

For application 1 of the improved alkaline purification treatment, the prior sulfite digestion ofwood is not lim itedto any particularme'thod orcondition's, such as choice of pulping base. Likeflise, thechemical properties of the pulp obtained from. the sulfite' digestion are not critical and any conventionalsulfite digestion treatme'ntmay be used, Normally, .howey f, the digestion will be. so carried out that a sc'r'eenable' pulp Will-be obtained which may; be chlorinated witho'utuneconomical. consumption of chlorine. Such 'sulfite digestion conditions will normally produce yields olf 40 to 50% of unrefined pulp, ba'sedoii the my initial wood. V r

S'ulfite cookin liquors to produce ascreenable pulp will" genera-11y contain about O15 to ,,1.5% of combined sulfur dio'ziide, i'epiesentin'g combination as neutral sulfite'; and 4% 190% "f free. sulfur dioxide.

Si'r'riila'rlythe in tion' is not limited to the use of, any

particular wdod in the v,sulfite digestion. Any softfoli hardwood which can be satisfactorily pulped the acid sulfite process may .be. used. Suitable woods include western hemleck, sprilee, southern pine, black gum, birch. maple, alder and ash.- z I V N Following sulfi'te digestion the pulp is washed by any conventional or' otherwise convenient manner such .as in' a blowpit or by drumwashers. "Ihewashed pulp in aqueous suspension is thench r inat'e d withelementjal chlorine according to conventional. practice for sulfi'te pulp. The amount of chlorine used is g'enerally siich that there will be: a s'rn allQeXcess; after the time allotted for reaetio'n'. Tli i;s' tre'atinent m y be carried outlat any convenient pulp consistency; lfiollowing chlorination, pulp is againwaslied'thoroughly with water using, for ex- In carryingout the pulp refining the. chlorinated and washed pulp is' digested, at a temperature above 140. C.,- ina liquor containing aniiirture of sodium carbonate and sodium sulfide, in which the sodium of the sodiumfsulfide preferably represents up to of the sodium reterit inthemixt'ure. t n Insofar as the properties the refined 'pulps are eoncerned, their'e i 'ttle dilfer ence overthe nursrmge of mr s i 1.99%..N 23i 99% t t 's-v ever; as-M11 be evident" from subsequent discussion, the

presence of more than 70% of the sodium as Na s in the mixture will result, unless the above-mentioned pretreatment is employed, in difliculties in the chemical recovery, especially as regards the smelting operation.

Most frequently the digestion will be carried out at a temperature Within the range 140 to 185 C. With a digestion time of the order of 45 minutes optimum results will generally be obtained at 150-175 C. With shorter times, higher temperatures may be used, even above 185 C. The operation may be conducted batchwise or continuously in suitable equipment. For continuous operation it is obvious that short times and high temperatures would be used. The consistency of the pulp slurry is not critical, but for economical reasons will be as high as can be conveniently mixed with the mechanical equipment available. A consistency of. to 20% will generally be used, The term consistency as used herein refers to the'weight of bone dry pulp per total weight of slurry in which it is contained, expressed as percent.

The total amount of Na CO and Na s used will be determined by the characteristics desired in the refined pulp. Usually from 6% to 20% of sodium salts, calculated as Na O and based on weight of bone dry pulp, will be used, more extensive refining being obtained with the higher proportions of chemicals. Since the chemicals are readily recovered, as hereinafter described, it is economically practical to use higher proportions of chemicals to pulp than has been possible in past sulfite pulp refining procedures using NaOH, where the NaOH could not be practically recovered.

The temperature used in the novel refining with sodium carbonate and sodium sulfide is considerably higher than that which would be used in refining with sodium hydroxide as hitherto practiced, for which a normal digestion temperature would be in the range of 85 -135 C. Actually the increased temperature used in the refining of the invention would, if the usual sodium hydroxide refining chemical were used, lead to uneconomical loss of yield without any further improvement in alpha cellulose. Surprisingly, however, using the mixture of sodium carbonate and sodium sulfide (rather than sodium hydroxide at a lower temperature) the pulp is produced not only in improved yields but has improved analytical values and has considerably better properties as regards use for acetylation or production of viscose yarn or cord.

The highly satisfactory refining in the invention at temperatures higher than that hitherto considered desirable may be due to the highly buffered character of the refining liquor. Both sodium carbonate and sodium sulfide have this property, and their mixtures possess it as well;

While the sodium carbonate-sulfide mixture of the invention gives results superior to those of the customary sodium hydroxide liquor used, it is not necessary that the digestion liquor be entirely free from sodium hydroxide. While, as described below, a cyclic process of pulp refining and recovery of refining chemicals is provided, any such process will have small losses of the sodium chemicals. This loss, which normally should not exceed may if desired be replaced by sodium hydroxide. Thus up to 15% of the sodium of the soda chemicals used may be added as NaOH without requiring additional chemical recovery equipment and without necessitating expensive causticizing and lime burning equipment. Also with up to 15% of the soda salts present as NaOH the solution will still retain its highly however, may be satisfactorily compensated for by using in place of or in addition to the usual hypochlorite bleaching at least one stage of bleaching with chlorine dioxide or acidified sodium chlorite.

The bleaching may be carried out at either high or low pulp consistencies and either batchwise or in continuous bleaching equipment.

Pulp refined by the invention, after bleaching in the manner described, in addition to having advantages of a high-alpha cellulose content and satisfactory brightness is generally characterized by a low content of impurities such as pentosans, lignin, etc. The product may be advantageously used for any of the customary uses for high alpha pulps.

The bleached refined pulp has particular advantages in the viscose process where, as compared with conventionally refined pulp, it gives yarn, cord or film not only of higher quality but also in higher yield.

Likewise the bleached refined pulp finds advantageous use in acylation and etherification processes. In acylation processes, especially as regards acetylation, the pulp reacts more rapidly than conventional high-alpha pulps and gives generally clearer solutions.

Other cellulose esters and ethers for whose preparation the pulp is suitable include cellulose propionate, cellulose aceto-propionate, cellulose aceto-butyrate, ethyl cellulose, methyl cellulose, carboxymethyl cellulose, hydroxyethyl cellulose, cellulose sulfate and the like.

Refining chemicals from the novel refining process may be recovered in a simple practical manner to provide a cyclic operation. Following digestion, the liquor associated with the pulp would be recovered from the pulp with a minimum of dilution, as for example by displacement on a rotary drum Washer. A substantial portion of this effluent liquor containing residual amounts of active chemicals and organic matter would be recycled directly to the digestion liquor makeup. This is advantageous from the standpoint of building up the organic solids content.

The remainder of this effluent liquor containing a high content of organic matter by virtue of recycling can be evaporated with substantial economy. Evaporation may be'in any conventional manner, as for example in a multiple effect vacuum evaporator, followed if desired by a direct contact with flue gas in a cascade evaporator. De-

; pending on viscosity and equipment characteristics the liquor will generally be evaporated to a solids content of 50-75%.

The evaporated liquor is then burned in a smelting recovery furnace under reducing conditions. The furnace will generally be equipped with water tubes to recover heat and generate steam and may be conventional krafttype recovery furnace.

Due to the relatively high ratio of inorganic to organic solids, it-may be necessary to supply auxiliary fuel such as fuel oil. This, however, cannot entirely be considered as an additional cost since the fuel value of this additional oil as well as a substantial portion of that of the organic solids in the liquor will be recovered in the steam generated.

In the simplest case Where the sodium in the sodium sulfide used in the mixture of refining chemicals does not amount to more than 70% of the total sodium content, the smelt produced by furnacing the evaporated effiuent will have a high carbonate content and thus have a low melting point. Thus it will flow freely from the furnace at moderate temperatures, avoiding excessive fume formation which would be impractical or very costly to recover.

The importance of this factor in the smelting process is illustrated by the following data on the fusion temperature'of mixtures of Na S and Na CO in various proportions. In column 1 the compositions are expressed as percent on the formula basis and in column 3 on the sodium oxide basis,

. d TABLE I Fusion temperature of Na S'Na' CO mixtures Percent of Total N310 Present as N MS Fusion Parent NazS in NsnS-NaiC o3 Mixture r oint,

H In actual smelting operations, the smelt will contain appreciable amountsof sodium sulfate and minor amounts of other sodium salts. These will tend to lower somewhat the fusion points given above for mixtures of pure sodiumcarbonate and sodium sulfite.

Actual fusion temperatures have been measured on two samples of smelts. In onesample in which the percentage of the total Na O presentas Na S was 45.0%, the Na CO was 45.0%, and Na SO was the fusion point was 738?, C. In another sample of smelt in which the composition was Na S 70 Na CO 20%, and NagSO 10% (Na O basis) the fusion point was 777" C.

Where the sodium sulfide content is suficiently high as to represent more than 70% of total sodium, it is possibleby a simple treatment to lower the sulfidity of the diluent so that after evaporation and furnacing a low melting point smelt may be still obtained. 7 This may be done.by treating the sulfide-containing effluent With a carbon dioxide-containing gas, preferably in a countercu rrent manner under pressureand at an elevated temperature. The carbonate solution is then subjected to vacuum steam stripping, preferably in such a manner that flashing occurs, .whereby hydrogen sulfide isevolved in concentrated, readily usable form. The stripped liquor having a lower sulfidity than originally may be evaporated and burned to produce a low melting point smelt with attendant advantages in furnace operation, e. g., permitting use of standard kraft-type recovery furnace, and by virtue of. its composition reducing the danger of explosion ,in dissolving the molten smelt. The concentrated hydrogen sulfide may be returned to the system by simply scrubbing the evolved gas with dissolved alkaline smelt solution thereby recoiistituting alkaline digestion liquor in a simple manner without major sulfur losses.

liven where the sodium of the sodium refining liquor represents less than 70% of the sodium, it will frequently be found advantageous to reduce the sulfidity of the refining efiduent by. such carbonation and vacuum steam stripping beforeev'aporation and burning in view of the attendant operating advantages andj minimizing of sulfur losses from both the evaporation and furnace.

As described previously, sodium hydroxide in amounts up to of the total Na O in the mixture may be used aspartial or complete sodium makeup. The relatively assesei mirror sulfur losses which will occur may be made up jby addition at a suitable point of such materials as sodium sulfate, sodium sulfide, hydrogen sulfide, elejm'ental sulfur or by addition to the waste refining efliuent of sodium or ammonium base acid sulfite waste liquor.

In any practical operation, especially when a cyclic process of refining and recovery is used, there will frequently be present in addition to the above-named active sodium compounds, substantial amounts of sodium sulfate and smalleramounts of sodium chloride, sodium thinate, :0 ot sc ium s s di q ial i m chemicals when present are relatively inert and do not enter into the useful reacuons iii the refining of pulp. They are therefore never considered in stating the .concentrations of active sodium chemicals in refining, neither with respect to the total proportion of active chemicals to pulp -nor with respect to the proportions of different chemicals in a mixture.

Where an auxiliary fuel is required for the proper combustion of concentrated effluent liquor, concentrated sulfite waste liquor may be used if desired in place of fuel oil. The concentrated sulfite waste liquor, however, should be either sodium or ammonium base liquor. This may result from digesting the wood with a solution containing sulfurous acid and either sodium or ammonium bisulfite. Alternatively where calcium base acid sulfite wood digestion liquor is employed, the waste liquor may be readily converted into suitable sodium base form by ion exchange.

Use of sulfite waste liquor as an auxiliary fuel hasa number of advantages including economy of fuel, increased heat recovery, minimizing of water pollution and recoveryof additional chemicals. It will be noted that use of'sulfite waste liquor will increase the ratio of sulfur to sodium in the liquor but undesirably high furnace operating temperatures due to the higher sulfidity of the smelt may be avoided by using the above-mentioned method of carbonation and vacuum steam stripping to reduce the sulfidity of the efiluent prior to evaporating and furnacing. ,7 a

The smelt will consist largely of sodium carbonate and sodium sulfide, together with smaller amounts of sodium sulfate, sodium thiosulfate, traces of sodium sulfite, and generally some sodium chloride as an impurity. The smelt solution after suitable clarification or filtration may be used for alkaline refining liquor. Makeup chemical may be added in suitable form such as sodium hydroxide, sodium carbonate, sodium sulfide or hydrogen sulfide. In addition, if the described process for sulfidity reduction of the refining efiluent has been used, at least part of the evolved hydrogen sulfide may be dissolved in the smelt solution. Chemical makeup through use of sodium sulfate or sodium or ammonia base sulfite waste liquor is more suitably added to the refining liquor before evaporation and/ or furnacing.

The fact that the alkaline refining liquor used in our novel process may be made up directly from the smellt and thus without the use of expensive lime burning and cansticizing equipment, required in recovery of alkaline chemicals in the kra' ft and soda pulping processes, is of great practical advantage.

If the sulfite wood digestion is accomplished by soda base acid sulfite liquor and the waste efiluent used as auxiliary fuel in combusting the waste alkaline refining liquor of the invention, a completely integrated cyclic process of acid sulfite pulping and alkaline refining becomes possible.

In such a process the smelt may not only be used directly makeup as alkaline refining liquor but also a portion may be converted to sodium bisulfite-sulfurous acid solution for use in the acid sulfite digestion of wood according to pending application, Serial No. 415,817, filed March 12, 1954, of Kenneth Russell Gray, Hartzell Lance Crosby, and JohnCharles Steinberg'. 4 I H h I V Alternatively, a completely cyclic acid sulfite pulping and alkaline refiningprocess is possible by using as before a portion of the smelt directly in making up alkaline refining liquor. The remaining portion of the smelt would be converted to sodium bis'ulfite-sulfurous acid wood digestion liquor by ion exchange according to United States Letters Patent 2,656,244, of Kenneth Russell Gray and 'Hartzell Lance Crosby.

EXAMPLE I Three batches of chlorinated and washed acid sulfite pulp from western hemlock wood were refined by digesting for 45 minutes in an agitated pot at temperatures of 125 C., 150 C., and 165 C. with liquor equivalent to 19.4% Na O (25% NaOH) consisting of a mixture in which one-half of the Na O was Na CO and one-half was Na- S. Following the refining digestions, the pulp batches were removed, washed with water, and dried.

Alkaline refining would normally be followed by bleaching. A bleaching step was, however, omitted in these tests in order to show more definitely the direct effect of the alkaline refining.

The refined and dried pulps were analyzed for alpha celluloseand tested for brightness by the following methods which are similar to those known and used in the art:

Alpha cellulose.Fraction of pulp insoluble in 18.0% NaOH solution, by a method closely similar to TAPPI Standard Method T203m.

Brightness.Percent reflectance of light at 457 mil-limicrons using a Beckman Model D. U. spectrophotometer equipped with reflectance attachment. Calibration of standards was such as to give results closely equivalent to those of TAPPI Standard Method T217m for measuring brightness with the General Electric reflection meter.

Using the above refining procedure and conditions and testing methods, the experimental values shown in Table II below were obtained:

EXAMPLE II V In order to demonstrate the improved acetylation properties of pulp refined by the invention, the following con- To the sample vial in -the water bath 15. ml. of the acetylating mixture are added from a pipette. The pulp and acid are mixed with the glass rod, which remains in the vial. The vials are stored in the water bath and the mixing repeated every 15-20 minutes. It is important to include a standard sample with each group of unknowns and to handle and agitate all samples alike.

As the pulp samples are acetylated by the mixture they dissolve continuously. The time required for substantial solution to take place and the relative clarity and residual undissolved fibers at the time of observation will indicate whether any of the samples is more or less reactive than the standard. An observation of color is also made.

As an example of pulp refined by the invention, a sample selected from the refining experiments described in Example I was tested by the above test. This sample was the one refined at 150 C. with a 50-50 mixture of Na CO and Na S. In the above described acetylation test it dissolved in 6.5 hours to give a solution with a very slight haze. In comparison, samples of sulfite pulp refined by conventional methods required 10-12 hours for substantial solution and even then contained many unreacted fibers.

EXAMPLE III In Table III are shown comprehensive analytical and acetylation test data on a series of samples of the same sulfite pulp refined at 160 C. with sodium carbonatesulfide mixtures in amount in each case equivalent to 19.4% Na O on the pulp (also equivalent to NaOH). The proportions of Na CO and Na S are, however, varied from 100% Na CO to 100% Na S. All proportions are on the basis of equivalents of Na O. The analytical data of Table III are also shown graphically in the attached graphs.

The analytical and test methods used, in addition to those already described in Examples I and II, included the determination of KOH solubility by measuring the fraction soluble on heating the pulp three hours at 100 C. in 10% KOI-I solution.

Results obtained with the refining experiments and test methods described follow in Table III:

TABLE III Effect of proportion of Na CO to Na S in refining with 19.4% total chemical as Na O at 160 C.

Refining Agent, Pet. Acetylation Test of N320 as- Refining Alpha KOH Bright'r Yield, Cellulose, Soluble, ness, Percent Percent Percent Percent Soln Clarity of Color of N e 00; Nags Time, Soln Sol'n Hrs.

venient and rapid laboratory test was used for comparing the acetylation reactivity of samples of pulp fibers:

Small specimens of the pulps to be examined are moistened with distilled water and dried in a circulating oven at a controlled elevated temperature to dry them under comparable conditions. (This step may be omitted if all samples have been previously dried in the same manner.) An accurately weighed sample of 0.5 gram of each pulp is torn into small bits and placed in a 35 ml. vial. A flattened glass rod is placed in the vial through a hole in the cap and the vial and sample set in a water bath at 20C.

The acetylating mixture is prepared by mixing 2.500 gram H 50 88.0 ml. acetic anhydride, and 175.0 ml. acetic acid. This mixture is unstable and should be freshly prepared every two days.

EXAMPLE IV In Table IV are given analytical and acetylation test data similar to Table III for a series of samples of hemlock sulfite pulp refined at C., in each case with equal parts of Na CO and Na s in terms of Na- O, and

TABLE IV 5. The method of producing high-alpha cellulose which comprises subjecting chlorinated and washed pulp from the acid sulfite digestion of wood to an alkaline refining digestion in a buflered aqueous alkaline liquor consisting of a mixture of sodium compounds consisting principally Efiect of amount of total chemical, using mixture of equal parts of Na CO and Na S at 160 C.

Acetylation Test Total Refining Alpha KOH Bright- N810, Yield, Cellulose, Soluble, ness, Percent Percent Percent Percent Percent Solution Clarity Time, of Color of Sol'n Hrs. Soln 7.8 83.0 94.7 9.9 55.8 8 Clear. Tan. 13. 6 77. 6 96. 3 5.1 54. 7. 2 do Do. 19.4 77.1 95.9 5.8 57.9 7.5 do Lt. Brown. 25. 2 75. 7 96. 4. 2 56. 9 7 do Do.

EXAMPLE V of sodlum sulfide 1n an amount not exceeding 70%,

The experiment which follows illustrates application of the invention to the production of a finished pulp suitable for use in the viscose process.

A sample of chlorinated hemlock sulfite pulp was refined by digesting it 45 minutes at 170 C. in a liquor containing sodium chemicals in the proportion of 9% Na O on the pulp basis, the sodium chemicals being present as Na CO 75% and Na S 25%. This pulp was bleached with sodium hypochlorite giving a product with the following analytical properties: Alpha 95.6%, Brightness 86.8%, KOH solubility 4.8%. These analytical qualities are well within the ranges generally considered suitable for use in the viscose process.

EXAMPLE VI The following experiments illustrate the application of the invention to the production of a finished pulp suitable for use in the acetate process.

A sample of chlorinated sulfite pulp from western hemlock was refined by digesting it at 16% consistency 45 minutes at 170 C. in a liquor containing sodium chemicals in the proportion of 9.1% Na O on the pulp bases, the sodium chemicals being present as Na CO 50% and Na S 50%. This pulp was bleached only with sodium hypochlorite and the finished product had the following analytical properties: Alpha 97.1%, Brightness 83.7%, KOH solubility 3.4%. The pulp was acetylated by a conventional test procedure in which it reacted smoothly to give an acetate of good clarity, color, and filterability.

I claim:

1. The method of producing high-alpha cellulose which comprises subjecting chlorinated and washed pulp from the acid sulfite digestion of wood to an alkaline refining digestion in an aqueous alkaline liquor consisting of a mixture of sodium compounds consisting principally of sodium sulfide up to 70%, and the remainder substantially all sodium carbonate, said solution containing from 6% to 20% of sodium compounds (all said percentages expressed as Na O), and said refining digestion being carried outwith a pulp consistency of from to and at a temperature of from 140 to about 185 C., and carrying out the alkaline refining digestion for a sufficient time to effectively remove pentosans, lignin and an appreciable amount of contaminating colored material, thereby producing high-alpha cellulose in good yield.

2. In the method of claim 1, using an alkaline digestion liquor in which the sodium sulfide and sodium carbonate are in approximately equal proportions and the digestion time is about forty-five minutes.

3. In the method of claim 1 carrying out the digestion at a temperature in the range of 150 C. to 175 C.

4. In the method of claim 1, using an alkaline digestion liquor in which the sodium sulfide expressed as Na O varies from 30% to 60%.

and the remainder substantially all sodium carbonate, said solution containing from 6% to 20% of sodium compounds (all said percentages expressed as Na O), said refining digestion being carried out with a pulp consistency of from 10% to 20% and at a temperature of from to about C., and carrying out the alkaline refining digestion for a sufiicient time to effectively remove pentosans, lignin and an appreciable amount of contaminating colored material, thereby producing highalpha cellulose containing at least 96% of alpha cellulose, good brightness, a yield of at least 77%, and potassium hydroxide solubility of not more than 6%.

6. In the method of claim 1, said aqueous alkaline liquor containing sodium hydroxide in an amount not exceeding 15%.

7. In the method of claim 5, said aqueous alkaline liquor containing sodium hydroxide in an amount not exceeding 15%.

References Cited in the file of this patent UNITED STATES PATENTS 1,387,441 Braun Aug. 9, 1921 1,566,118 Rawling Dec. 15, 1925 1,640,853 Richter Aug. 30, 1927 1,651,665 Bradley et al. Dec. 6, 1927 1,802,575 Richter Apr. 28, 1931 1,822,125 Blodgett et al. Sept. 8, 1931 1,830,421 Bradley et a1 Nov. 3, 1931 1,859,847 Rue et al May 24, 1932 1,860,848 Bradley et a1. May 31, 1932 1,870,650 Richter Aug. 9, 1932 1,880,046 Richter Sept. 27, 1932 1,921,539 Richter Aug. 8, 1933 1,973,557 Bradley et al Sept. 11, 1934 1,974,751 Richter Sept. 25, 1934 2,018,490 Jones et al. Oct. 22, 1935 2,249,174 Richter July 15, 1941 2,320,294 Palmrose et al. May 25, 1943 2,694,631 Richter et al Nov. 16, 1954 FOREIGN PATENTS 480,404 Canada Jan. 22, 1952 27,733 Denmark June 6, 1921 4,984 Great Britain of 1880 OTHER REFERENCES Richter, Ind. & Eng. Chem; February 1931; pp. 138 and 139.

Aronovsky, Paper Ind.; September 1934; pp. 413-418.

Yorston, Dominion Forest Service Bull. 97, Ottawa, Canada (1942), pp. 32, 41, 46, 53 and 55.

Wood Chemistry by Wise and Jahn, 2nd ed. (1952), publ. by Reinhold, New York, N. Y.; pp. 1055, 1056 and 1057. 

1. THE METHOD OF PRODUCING HIGH-ALPHA CELLULOSE WHICH COMPRISES SUBJECTING CHLORINATED AND WASHED PULP FROM THE ACID SULFITE DIGESTION OF WOOD TO AN ALKALINE REFINING DIGESTION IN AN AQUEOUS ALKALINE LIQUOR CONSISTING OF A MIXTURE OF SODIUM COMPOUNDS CONSISTING PRINCIPALLY OF SODIUM SULFIDE UP TO 70%, AND THE REMAINDER SUBSTANTIALLY ALL SODIUM CARBONATE, SAID SOLUTION CONTAINING FROM 6% TO 20% OF SODIUM COMPOUNDS (ALL SAID PERCENTAGES EXPRESSED AS NA2O), AND SAID REFINING DIGESTION BEING CARRIED OUT WITH A PULP CONSISTENCY OF FROM 10% TO 20% AND AT A TEMPERATURE OF FROM 140* TO ABOUT 185*C., AND CARRYING OUT THE ALKALINE REFINING DIGESTION FOR A SUFFICIENT TIME OF EFFECTIVELY REMOVE PENTOSANS, LIGNIN AND AN APPRECIABLE AMOUNT OF CONTAMINATING COLORED MATERIAL, THEREBY PRODUCING HIGH-ALPHA CELLULOSE IN GOOD YIELD. 